the "relay" near the brake booster isnt a relay, it is the boost
overload protection valve.

your tach most likely has a sensor and a sending unit on and
in front of the harmonic balancer.
relays are just big switches, activated by smallwires/small
charges; putting out larger charges through larger wires. 12v
current loses its forse over distances, so you keep the heavy
current runs as short as possible.

i thought most trap oxidizers in california had been removed
via recall. they looked like a roll of joined metal balls at the
exhaust ports.

don "the electrical idiot"